Well, I wouldn't do anything much to an existing floor. Sure, make it heavy if you are pouring something new or maybe have a really heavy machine.
I've got a whole shop full of machinery weighing 6000 to 12,000 lbs each, sitting on a 4 to 6 inch slab. There were cracks in the floor when I bought the building 15 years ago, and that would be due to settling and poor compaction I suppose. But setting machines on the floor has not really caused any new, ill effects that I can see. Maybe I'm just lucky.
The compressive strength of concrete is high enough to give a good safety factor, since it is really just transmitting the weight to the fill beneath the building.
I use plywood pads beneath the steel 'feet' that come with most of my machines. I figure a piece of plywood will contour a bit better to the floor than a piece of steel, and help distribute the load better. My lathes do not suffer going out of level to any degree, summer or winter, unless I move them around. Just test them a few times after the installation to see if everything is ok.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)